1. Field of the Invention
The present invention relates to a method for measuring the distance between a sensor electrode and a workpiece.
2. Description of the Relevant Art
It is already known in the case of machine tools, in particular in the case of laser machine tools, for the distance between a workpiece and a tool head for machining the workpiece to be measured in a capacitive way. For this purpose, it is customary for the tool head to be fitted with a sensor electrode which forms together with the workpiece a measuring capacitor whose capacitance depends on the distance between the sensor electrode and workpiece. The sensor electrode then supplies a sensor signal from which it is possible, for the purpose of controlling the machine tool, to derive the capacitance of the measuring capacitor, and thus the distance to be measured between the sensor electrode and workpiece.
By monitoring the sensor signal, it is therefore possible for the tool head to be positioned accurately relative to the workpiece in order to be able to machine the workpiece in a suitable way. The positioning is performed in this case via a control device which receives the measured distance, determined from the sensor signal, as actual value and compares it with a prescribed desired value in order to control the tool head as a function of the result of comparison.
However, in the case of machine tools such as flame cutting machines and, in particular, laser machine tools, there is the problem that as a workpiece is being processed a plasma which acts essentially like an ohmic resistance connected in parallel with the measuring capacitor is formed between the sensor electrode and the workpiece. Such a plasma, which is produced, for example, below a laser cutting nozzle when a certain cutting speed is reached during welding or cutting work has a falsifying effect on the distance measurement.
In order to exclude the influence of a plasma between the sensor electrode and workpiece on the distance measurement, use is made in conventional distance-measuring methods of the LC-oscillator principle, in which the measuring capacitor forms, together with an inductive resistor connected in parallel, a resonant circuit whose frequency, which depends on the measuring capacitance, is monitored in order to determine the distance to be measured. A plasma acting essentially only as an ohmic resistance in this case influences only the amplitude of the oscillation of the resonant circuit, but not the frequency thereof.
A problem in this method is that stray capacitances must be kept very low in order to achieve the measuring accuracy required for reliable control of the machine tool. Complicated insulation measures are required to reduce the stray capacitances.
DE 40 20 196 A1 discloses a further capacitive distance-measuring method in which a measuring capacitor is fed with a constant alternating current so that the measuring voltage tapped at the sensor electrode of the measuring capacitor depends exclusively on the impedance of the measuring capacitor. As long as no plasma exists between the sensor electrode and the workpiece during machining of the workpiece, the impedance is formed virtually exclusively by the capacitive resistance of the measuring capacitor, with the result that the measuring voltage is proportional to the distance between the sensor electrode and workpiece. However, if a plasma occurs, there is an ohmic resistance parallel to the capacitance of the measuring capacitor which influences the impedance of the measuring capacitor. In this case, the plasma can lead to the reduction in the impedance of the measuring capacitor such that the sensor signal virtually collapses, and the control device erroneously returns much too small a distance.
It is possible in the case of this known method for short-term disturbances owing to a plasma to be masked out electronically, or for the influence of a plasma cloud to be essentially excluded by geometrical measures when designing the sensor electrode. However, it has emerged that during working the sensor signal is in some cases permanently disturbed by a continuously present plasma, and that the geometrical measures in the design of the sensor electrode worsen the spatial resolution of the measurement.
Starting therefrom, it is the object of the invention to provide a method for measuring the distance between a sensor electrode and a workpiece which eliminates the influence of a plasma between the sensor electrode and workpiece on the distance measurement.
Thus, according to the invention in a method for measuring the distance between a sensor electrode and a workpiece in which the sensor electrode forms with the workpiece a measuring capacitor through which an alternating current flows, a voltage present at the sensor electrode is tapped as measuring voltage. The real part and imaginary part of this measuring voltage are determined, in order to determine therefrom the distance to be measured.
In this way, the capacitance or the reactance, dependent on the distance between the workpiece and sensor electrode, of the measuring capacitor, and thus the distance itself, can be calculated independently of the plasma impedance even in the case of a varying measuring current. It is thereby possible not only to eliminate the influence of a plasma between the sensor electrode and workpiece on the distance measurement, but it is also no longer necessary to provide an additional current source for generating an alternating current with constant amplitude. The method according to the invention thus does not require electronic and/or geometrical measures for masking out or suppressing disturbing influences of the plasma, nor additional current sources.
The invention thus adopts a completely new approach by accepting for the distance measurement, in addition to changes in the measuring-current amplitude, the production of a plasma and the influence thereof on the impedance of the measuring capacitor and, instead of this, selecting the measuring frequency used such that at this frequency the plasma acts virtually as a pure ohmic resistance. This renders it possible to use the real and imaginary parts of the measuring voltage to determine the measuring capacitance, in order then to use appropriate subsequent calculations or calibrations to obtain the distance or a signal indicating the distance.
Since the properties of a plasma between the measuring electrode and the workpiece depend on the current parameters of the respective machining, it is provided in the case of an advantageous development of the invention that a signal corresponding to the electric properties, in particular the resistance of the plasma between the sensor electrode and workpiece is determined from the real and imaginary parts of the measuring voltage. This signal can then be used to monitor and control the respective machining operation, and thus for quality assurance.
In an expedient development of the invention, it is provided that in order to determine its real part and imaginary part, the measuring voltage is combined with a first and a second AC voltage which are mutually phase-shifted by a quarter period.
It is possible in this case that in order to determine its real part and imaginary part, the measuring voltage is multiplied by a first alternative voltage and a second AC voltage phase-shifted with respect thereto by a quarter period, preferably by a cosinusoidal or sinusoidal AC voltage, respectively. Another advantageous possibility consists in that in order to determine its real part and imaginary part, the measuring voltage is subjected to a first and a second synchronous rectification, respectively.
The method can be carried out with particular ease if for the purpose of the first synchronous rectification use is also made of the same AC voltage which serves to generate the measuring voltage, while for the purpose of the second synchronous rectification use is made of the same AC voltage phase-shifted by a quarter period with respect thereto.
For further calculations, the components of the measuring voltage obtained by multiplication or synchronous rectification are then expediently freed from AC voltage components by low-pass filtering, in order to obtain voltage signals corresponding to the real and imaginary parts of the measuring voltage.
A particularly expedient development of the invention is distinguished in that using the real and imaginary parts of the AC voltage used to generate the measuring voltage the measuring capacitance or the reactance of the measuring capacitor is determined from the voltage signals representing the real and imaginary parts by calculating the voltage divider formed by the reference resistor and measuring capacitor.
The invention not only has the advantage of a distance measurement which can be continued with high accuracy for controlling a tool head of a machine tool even given the occurrence of plasma formation, but in addition it also permits monitoring of the machining operation if using the real and imaginary parts of the AC voltage used to generate the measuring voltage the plasma impedance of the measuring capacitor (16) is determined from the voltage signals representing the real and imaginary parts by calculating the voltage divider formed by the reference resistor and measuring capacitor. As a result, it is possible, on the one hand, to detect the existence or non-existence of a plasma and the intensity of a plasma located between the sensor electrode and workpiece, and to use it to monitor the machining operation.
For example, the acceptable execution of laser cutting can be monitored by observing the plasma. During normal, acceptable laser cutting, a plasma forming in the process is essentially blown away by the ensuing cut, with the result that only a thin plasma with a high ohmic resistance and therefore a weak influence on the distance measurement is present between the sensor electrode and the workpiece. If the acceptable cutting breaks down, however, for whatever reason, the plasma density rises between the sensor electrode and workpiece, with the result that at the same time the resistance of the plasma drops sharply, and this can be established straight away if the plasma impedance is continuously determined. Such a rise in the plasma conductivity therefore indicates a fault in the laser cutting. Monitoring the plasma impedance therefore permits faults to be detected early, and suitable countermeasures to be taken.
In order to increase the accuracy of the distance measurement and, if appropriate, to enhance the plasma monitoring, it is provided according to the invention that the measuring line to the sensor electrode is actively screened, in which case the measuring voltage tapped via the measuring line is expediently applied to the screen of the measuring line via an impedance transformer. It is possible in this way for stray capacitances, which occur in parallel with the measuring capacitance, to be substantially reduced, thereby not only achieving a simpler technical design, but also simplifying the voltage divider and, consequently, its calculation.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.